1. Field of the Invention
The present invention relates generally to measurement-while-drilling systems. More particularly, the present invention relates to downhole data transmission in a measurement-while-drilling system. Most particularly, the present invention relates to a method and apparatus for processing noise and reflections from a surface-received waveform to yield a more accurate depiction of a signal transmitted from downhole.
2. Description of the Related Art
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole commonly is referred to as "logging." Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled and can be performed by several methods. In conventional oil well wireline logging, a probe or "sonde" is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. A wireline sonde may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors. The sonde typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface and to control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed, or tripped, from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers possibly must make decisions (such as the direction to drill, etc.) without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be questionable. As one skilled in the art will understand, wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. In addition, the borehole shape may begin to degrade, reducing the accuracy of the measurements.
Because of these limitations associated with wireline logging, there recently has been an increasing emphasis on the collection of data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections "real-time", as necessary, to optimize drilling performance. For example, the driller may change the weight-on-bit to cause the bottomhole assembly to tend to drill in a particular direction. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Techniques for measuring conditions downhole, and the movement and location of the drilling assembly contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the position of the drilling assembly while the bottomhole assembly is in the well. The measurement of formation properties during drilling of the well by LWD systems improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation in which the borehole is traversing.
Referring to FIG. 1, there is illustrated an LWD system. A well bore or borehole 100 contains a drill string or drill pipe 110 including a hollow center region 111, and defines an annulus 130 (the region between the outside of the drill string and periphery of the borehole). Also shown are stand pipe 115 including an area of curvature 112, drill bit 125, and transmitter 140. Stand pipe 115 connects above the earth's surface (or rig floor) 117 to desurger 160, pressure transducer 150, signal processor 155 through a transmission line 158 and transducer 150, a mud pump 120, and drill string 110. Desurger 160 contains a high pressure area 162 and a rubber interface 164. Drill bit 125 attaches to drill string 110 at the lower end of the drill string. Transmitter 140, part of a bottom hole assembly (not shown in its entirety), is located near the bottom 111 of the drillstring, proximate to drill bit 125.
Typically, a pit at the surface of the earth (not shown) contains drilling fluid or mud 122. Mud pump 120 forces the drilling fluid 122 into region 111 of the drill string, where it flows in a downward direction as indicated by arrow 124. Eventually, it exits the drill string via ports in the drill bit 125 and circulates upward via annulus 130, as indicated by flow arrows 126. The drilling fluid thereby lubricates the bit and carries formation cuttings to the surface of the earth. The drilling fluid is returned to the pit for re-circulation.
Acoustic transmitter 140 generates an information signal 170 representative of measured downhole parameters. Information signal 170 typically is an acoustic pulse signal that travels in an upward direction 175 along the mud column (inside region 111) at the speed of sound. One suitable type of acoustic transmitter employs a device known as a "mud siren" which includes a slotted stator and a slotted rotor that rotates and repeatedly interrupts the flow of drilling fluid to establish a desired acoustic wave signal in the drilling fluid. Such a mud siren conventionally operates at carrier frequencies ranging from 12 Hz to 24 Hz, with data transmission rates ranging from 3 bits per second (bps) to 6 bps. Other acoustic transmitters are also known. These alternate transmitters have lower data transmission rates around 1 bps and frequency spectrum in the range of mud pump noise, so that signals are more difficult to recover at transducer 150. Driving electronics (not shown) in the bottom hole assembly include a suitable modulator such as a phase shift key (PSK) modulator, a frequency shift key modulator (FSK), or an amplitude modulator (AM), each of which conventionally produces driving signals for application to transmitter 140. Pressure transducer 150 receives the acoustic mud wave 170 at an uphole location, such as at the surface of the earth. Pressure transducer 150, which is, for example, a piezoelectric transducer, converts the received acoustic signals to electronic signals. Transducer 150 outputs the received waveform to signal processor 155 via the transmission line 158. Signal processor 155 operates to process and demodulate the received signals.
The presence of extraneous signals and noise along the mud column complicates the interpretation by signal processor 155 of an acoustic signal 170 received at pressure transducer 150. For example, the noise generated by the drilling assembly, the flow of mud through the drillstring, the grinding of the drilling components, and other mechanical and environmental noises make it difficult for the pressure transducer 150 to receive the transmitted acoustic wave 170 and to isolate the data contained in the acoustic waveform from extraneous noise. In particular, the operation of the mud pump 120, and specifically the action of its pistons (not shown), generates significant acoustic noise.
In addition to noise, reflection of acoustic signals is a significant problem. Upon the generation of an acoustic signal 170 by transmitter 140, the acoustic signal travels upward 175 toward and past pressure transducer 150. This acoustic signal may then reflect off mud pump 120, desurger 160, or both. The reflected signals rebound and travel in the opposite direction, namely downward 124 back towards the acoustic transmitter 140. As such, it is particularly difficult for the pressure transducer 150 to distinguish whether a received wave form is an upcoming waveform, a downgoing waveform, or a combination waveform. Indeed, if pressure transducer 150 is located at a node where the upcoming and downgoing signals cancel, pressure transducer 150 will erroneously indicate that no signal has been sent by downhole transmitter 140. Even if pressure transducer 150 is not located at such a node, the presence of the reflected, downgoing acoustic signals from the mud pump and desurger complicate analysis of the received waveform. These effects are in addition to the noise created by the mid pump's pistons.
Mud pump 120 acts as a solid reflector to the acoustic signal 170. As such, upon arrival at the mud pump 120, signals from transmitter 140 are reflected back in the opposite direction. These reflected signals maintain approximately the same amplitude and frequency of the original signals 170, but normally have different phases. Although there is not any appreciable difference for the range of transmission speeds currently available, at low frequencies, these reflected signals attenuate more slowly, and so the mud pump reflection phenomenon is more significant. As such, present transmission frequencies all are "low" enough that this is a significant effect. The exact nature of the signal reflected off a mud pump is difficult to predict for a myriad of reasons. For example, there are a variety of different mud pump manufacturers, and mud pump seals may be in different states of wear from mud pump to mud pump.
Desurger 160 includes a high pressure area 162 and a rubber interface 164. Rubber interface 164 and high pressure area 162 combine to absorb energy from the system and thereby dampen many of the transients present in the drilling system. Such absorption of energy helps to ensure that the system becomes and stays stable. Stability in the system minimizes the chances that energy from the mud pump pistons cracks or otherwise damages the piping of the drill string 110. However, this advantageous feature of desurger 160 is mitigated by the desurger's propensity to reflect and distort acoustic waves. Acoustic signals, and particularly lower frequency acoustic signals, reflect or bounce off desurger 160. As such, the desurger reflection phenomenon is more significant at lower frequencies. The character of the signals 190 reflected from desurger 160 is generally more complex than the character of the mud pump signal reflection 180. A signal 190 reflected from desurger 160 not only has an altered phase when compared to the transmitted signal 170, but also may have a significantly different amplitude and frequency distributions. Once again, the exact nature of the reflected waveform 190 is difficult to predict.
Therefore, pump noise and reflected signals often corrupt the signal 170 received by transducer 150. Signal corruption degrades the signal-to-noise ratio of the waveform received by transducer 150. As is well known, a lower signal-to-noise ratio decreases the reliability of the received waveform, and thus limits the data transmission rate. Conversely, a higher signal-to-noise ratio increases the reliability of the received waveform and allows an increased data transmission speed.
As such, an invention is needed to eliminate much of the noise and the reflected signals from the mud stream so that transducer 150 may better retrieve the data encoded signal 170 from the mud stream. Ideally, such an invention would not depend on knowledge of the waveform corresponding to the noise and reflected signals. Preferably, such an invention could be installed at the surface, where the acoustic signals are normally received, so as not to complicate the downhole assembly. In addition, such a solution preferably could be integrated into a known MWD system, so as to minimize costs.